1. Field of the Invention
The present invention concerns a bearing assembly for a gearbox shaft, for example, a differential drive shaft with a bevel pinion of a vehicle transmission, in which the bearing assembly is subject to bending forces and able to absorb axial thrust loads.
2. Description of the Related Art
Transmission drive lines, especially those in agricultural and other utility vehicles, frequently have gearbox drive shafts that transmit their rotational speed and their torque through a bevel gear to a further gearbox shaft. Usually these gearbox shafts are differential drive shafts.
Gearbox shafts that are unilaterally loaded by helical gears or bevel gears deflect with increasing gear loads. Helical gears simultaneously impose axial loads on the gearbox shaft that also increase with increasing gear load. These central axial forces are transmitted by thrust bearings to the bearing housing. If the gearbox shaft is inclined in the vicinity of the thrust bearings (due to deflection of the shaft), the thrust bearings can be subjected to an asymmetrical axial load with respect to their circumference, which could severely reduce the bearing endurance life of cylindrical roller thrust bearings.
Generally, differential gearbox shafts develop very high radial and axial forces at the bevel gear, which are absorbed in known designs by tapered roller bearings, cylindrical roller bearings, ball bearings, angular contact ball bearings or combinations of these bearings.
If transmission of higher power is desired, the rotational speed of the gearbox shafts generally is increased to avoid enlarging the dimensions of the gearbox excessively. This is usually accomplished in the multi-speed gearbox which drives the differential shaft. Using such methods, the resulting loads on the shafts and gears can be kept low. The higher rotational speeds, however, lead to increasing encroachments upon the rotational speed limit of the bearing, as well as to rising frictional horsepower with increasing speed.
Current differential gearbox input shafts usually are supported in tapered roller bearings, which reach their carrying capacity only after a higher axial preload is applied. At higher speeds, however, this preload leads to sharply growing frictional horsepower losses, which are no longer acceptable.
In addition, the increasing demand for simple and rapid initial assembly, as well as simple disassembly and reassembly with simple tools in the event of an overhaul of the vehicle transmission, can no longer allow the selection of tapered roller bearings. The precise control of preload they require is possible only with costly gaging and at the minimum requires the employment of trained personnel. However, this is not always assured, especially if the vehicles is exported into less technically developed countries.
The alternative selection of radial cylindrical roller bearings in combination with ball bearings or double angular contact bearings is not always favorable, as revealed by the German magazine "Walzlagertechnik" (Rolling Contact Bearing Technology) FAG 1984/2 Porsche Sports Racing Cars 956--Peak Performance Even for Rolling Contact Bearings, page 28. For one, the selection of a radial cylindrical roller bearing in conjunction with a ball bearing may not even be possible for heavy loads, since such a structure requires greater space, which simply is not available. For another, the selection of a radial cylindrical roller bearing together with an angular-contact ball bearing results in an undesirable cost increase.
The German publication from FAG Kugelfischer Georg Schaefer & Co. Schweinfurt,"The Design of Rolling Contact Bearings", Publication Number 00200DA, 1970, page 59 reveals a truck pinion bearing arrangement of the aforementioned type where the radial forces are absorbed by two radial cylindrical roller bearings and the axial forces are separately absorbed by two angular contact ball bearings. This bearing arrangement should result in low friction, temperature and wear with a high rotational speed limit and stiffness. In addition, the pinion can be located axially by inserting shims between the angle bushing clamped by the angular contact ball bearings and the gearbox housing, so that it can be brought to the correct position relative to the ring gear. However, angular contact ball bearings demand relatively high manufacturing requirements resulting in higher costs.
Page 15 the same publication reveals a bearing arrangement for a lathe spindle that combines a radial cylindrical roller bearing with two axial ball thrust bearings. The endurance life of axial ball thrust bearings decrease rapidly with increasing shaft deflection in the vicinity of the bearing. This is tolerable in a lathe, since a lathe spindle is not allowed to deflect and hence is relatively large in diameter. It would not be tolerable in a differential shaft.
In gearbox shafts which are designed to be relatively thin, for reasons of weight and cost, and that deflect under the various forces applied (deflection can occur in gearbox shafts, for example, that are subject to unilateral gear loads), the axial thrust bearings are loaded non-uniformly around their circumference and consequently must be designed to be correspondingly massive. The deflection of a gearbox shaft can lead to increased loads on ball and roller thrust bearings, whose endurance life rapidly decreases with increasing angular deflection. In conventional bearing arrangements, such as tapered roller bearings, or in arrangements with angular contact ball bearings, shaft deflections do not have as much of a detrimental effect as with simple ball or roller bearings, since the former admit of a greater angular error compared to the latter.